It has been previously proposed to use an electric power steering system for steering the steerable wheels of a vehicle by actuating a steering rod connected to the wheels at either end via a tie rod and a knuckle arm, along its axial direction. For instance, according to Japanese utility model laid open publication No. 1-179070 (two of the inventors of the present application are also inventors of this Japanese utility model application), an electric motor is disposed around a steering rod and the rotational movement of the electric motor is converted into the axial movement of the steering rod via a ball and screw mechanism.
In such a structure, it is necessary to prevent the rotation of the steering rod around its axial center in order to achieve a precise control of the axial movement of the steering rod. According to the aforementioned Japanese utility model application, it was proposed to provide a rotation preventing shaft which extends in parallel with the steering rod at a certain distance from the steering rod and is slidably received in a guide fixedly provided in the casing of the electric power actuator. However, this proposal did not offer a complete solution to the problem of preventing the turning of the steering rod around its axial center because a certain clearance was inevitable between the guide and the rotation preventing shaft.
In such an electric power steering system, it is highly desirable to minimize its external dimensions as it is typically mounted in a limited space in a lower part of the vehicle body, and in particular it is desired to reduce its lateral dimensions as much as possible. According to the aforementioned utility model application, a spring-loaded neutralizing unit 12 as well as a differential transformer 46 is attached to one side of the casing of the electric power actuator, and this substantially adds lateral dimensions to the electric power steering system. If the spring-loaded neutralizing unit is mounted on an axial end of the power steering system as proposed in Japanese utility model laid opening publication No. 63-164082 for application to a hydraulic power steering system, its longitudinal dimension is undesirably increased.
Yet another problem in such an electric power steering system is the need for a displacement sensor for measuring the output displacement of the power steering system, typically given as the displacement of a steering rod disposed at the output end of the power steering system, which takes up a substantial space in the system. And, further, in a front and rear wheel steering vehicle, such sensors are required for the front wheels and the rear wheels, and they must have widely different dynamic ranges because the rear wheels are steered by substantially smaller angles than the front wheels. The need for such two different steering angle sensors inevitably contributes to an increase in the manufacturing cost of the steering system for the front and rear wheel steering vehicles.